Currently, the output impedance, which is generally a few Ohms, of power amplifiers in cellular phones is adapted to the impedance of the phone's antenna, which is typically 50 Ohms, with a lumped-element inductor-capacitor (LC) matching network. Increasingly, cellular phones must operate in multiple modes and across a wider range of frequencies. Also, improved efficiency and decreased size are increasingly desired for the phone's radio frequency (RF) functions, including the matching networks, so that additional services desired by customers can be added to the phone without compromising usability. LC power amplifier matching networks have three principal disadvantages that affect their ability to meet these needs: they are generally lossy, generally large, and generally operate effectively only over a limited bandwidth (approximately 10%).
At low frequencies, transformers are widely employed to implement high impedance ratio matching with low insertion loss. Typically, transformers employ some type of ferrite material, with high electrical permeability, to enhance the magnetic coupling between two electrical coils or inductors. However, at cellular frequencies of generally approximately 1 GigaHertz (GHz) and above, high permeability materials typically are associated with high RF losses, and thus are generally unsuitable. A type of non-isolating transformer, generally referred to as transmission-line (TL) transformers, is generally well known and employs both electrical and magnetic coupling, rather than magnetic coupling only. These devices may achieve low insertion loss at cellular frequencies, but in comparison to traditional magnetic or flux coupled transformers, they have two general disadvantages: they are physically very large, and they are not electrically isolated.